Exothermic & Endothermic Reactions

2 ORGANISATION OF THE ORGANISM

YOUR NOTES

6 CHEMICAL ENERGETICS

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CIE IGCSE Chemistry Revision Notes

CIE IGCSE Chemistry Resources

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CONTENTS:

6.1 CHEMICAL ENERGETICS

6.1 .1 ENERGETICS OF A REACTION

6.1.2 ENERGY TRANSFER

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6.1 CHEMICAL ENERGETICS

6.1.1 ENERGETICS OF A REACTION

Exothermic & Endothermic Reactions

EXAM TIP

To help you remember whether a chemical system is exothermic or endothermic, in EXothermic reactions heat Exits the system and in ENdothermic reactions heat ENters the system.

Exothermic reactions always give off heat and they feel hot, whereas endothermic reactions take heat in and they feel cold.

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6.1 .1 ENERGETICS OF A REACTION cont...

EXTENDED ONLY

Bond Breaking & Bond Forming

Endothermic & Exothermic reactions

• Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed

Endothermic

• If more energy is absorbed than is released, this reaction is endothermic

• More energy is required to break the bonds than that gained from making the new bonds

• The change in energy is positive since the products have more energy than the reactants

• The symbol ∆H (delta H) is used to show the change in heat energy. H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction

• Therefore an endothermic reaction has a positive ∆H

Breaking chemical bonds requires energy which is taken in from the surroundings in the form of heat

Exothermic

• If more energy is released than is absorbed, then the reaction is exothermic

• More energy is released when new bonds are formed than energy required to break the bonds in the reactants

• The change in energy is negative since the products have less energy than the reactants

• Therefore an exothermic reaction has a negative ∆H value








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EXTENDED ONLY cont...

Making new chemical bonds releases energy which radiates outwards from the reaction to the surroundings in the form of heat

Energy level diagrams

• These are graphical representations of the heat changes in chemical reactions (see above)

• The enthalpy of the reactants and products is displayed on the y-axis

• The reaction pathway is shown on the x-axis

• Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)

Enthalpy change during an exothermic reaction explanation:

• During an exothermic reaction, energy is given out

• This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (∆H) is negative

• This is represented on the energy-level diagram above with a downwards arrow as the energy of the products is lower than the reactants

Enthalpy change during an endothermic reaction explanation:

• During an endothermic reaction, energy is absorbed

• This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (∆H) is positive

• This is represented on the energy-level diagram above with an upwards arrow as the energy of the products is higher than the reactants








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EXTENDED ONLY

Calculating the Energy of a Reaction

Energy of reaction calculations

• Each chemical bond has a specific bond energy associated with it.

• This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.

• This energy can be used to calculate how much heat would be released or absorbed in a reaction.

• To do this it is necessary to know the bonds present in both the reactants and products.

Method

• Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.

• Add together the bond energies for all the bonds in the products – this is the ‘energy out’.

• Calculate the energy change: Energy change = energy in – energy out

Equation

Energy change = Energy needed in - Energy given out

Example: An exothermic reaction

Hydrogen and chlorine react to form hydrogen chloride gas:

H2 + Cl2 → 2HCl

The table below shows the bond energies relevant to this reaction:

BOND BOND ENERGY (kJ / mole)

H - H 436

Cl - Cl 243

H - Cl 432








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Example: An Endothermic reaction

Hydrogen Bromide decomposes to form Hydrogen and Bromine:

2 x ( H - Br ) → H - H + Br - Br

The table below shows the bond energies relevant to this reaction:

Energy In = 2 x 366 = 732 KJ / Mole

Energy Out = 436 + 193 = 629 KJ / Mole

Energy Change = 732 - 629 = +103 KJ / Mole

*The energy change is positive, showing that energy is taken in from the surroundings so is an endothermic reaction

EXTENDED ONLY cont...

Energy In = 436 + 243 = 679 KJ / Mole

Energy Out = 2 x 432 = 864 KJ / Mole

Energy Change = 679 – 864 = -185 KJ / Mole

*The energy change is negative, showing that energy is released to the surroundings so it is an exothermic reaction.

BOND BOND ENERGY (kJ / mole)

H - Br 366

H - H 436

Br - Br 193








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6.1.2 ENERGY TRANSFER

Fuel, Combustion, Hydrogen

Energy from Fuels

• A fuel is a substance which releases energy when burned

• When the fuel is a hydrocarbon then water and carbon dioxide are produced in combustion reactions

• Propane for example undergoes combustion according to the following equation:

• C3H8 + 5O2 → 3CO2 + 4H2O ∆H = -2219 kJ/mol

• The efficiency of a fuel refers to how much energy is released per unit amount

• We can measure the efficiency of fuels by calorimetry

• A known mass of the fuel is combusted and used to heat up a known mass of water to calculate its heat of combustion

• Different fuels heat the water by different amounts and they can be analysed and compared in this way

Calorimetry experiment

EXAM TIP

For bond enthalpy questions, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.

The reaction thus becomes: H-H + Cl-Cl → H-Cl + H-Cl








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6.1.2 ENERGY TRANSFER cont...

Diagram showing the calorimetry experiment for combustion

Method:

• Using a measuring cylinder, put 100 cm3 of water into a copper can

• Measure and record the initial temperature of the water

• Fill the spirit burner with test substance and measure and record its mass

• Place the burner under the copper can and light the wick

• Stir the water constantly with the thermometer and continue heating until the spirit burner burns out

• Measure and record the highest temperature of the water








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6.1.2 ENERGY TRANSFER cont...

Hydrogen as a fuel

• Hydrogen is used in rocket engines and in fuel cells to power some cars

• Hydrogen has a series of advantages and disadvantages regarding its use as a fuel

• Advantages:

• It releases more energy per kilogram than any other fuel (except for nuclear fuels)

• It does not pollute as it only produces water on combustion, no other product is formed

• Disadvantages:

• Expensive to produce and requires energy for the production process

• Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurised containers)

Radioactive Isotopes as Fuels

• Uranium-235 undergoes decay and gives off heat energy which nuclear power stations harness

• The heat it produces is used to heat water to steam, which in turn is used to power turbines to generate electricity

• Nuclear fuel energy is clean as it does not produce pollutants such as CO2 or oxides of nitrogen or sulfur

• But nuclear power plants are expensive to build and maintain as well as being potentially dangerous in the event of an accident as radioactive materials may be released

The nuclear fission of a large nucleus of uranium-235 into smaller daughter nuclei

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EXAM QUESTIONS

QUESTION 2?Which of the following processes is endothermic?

A Reacting sodium with water.

B The use of petrol in an engine.

C Distilling crude oil.

D Burning fossil fuels.

QUESTION 1?A student adds a small amount of ammonium chloride to a beaker of water. The temperature of the water decreases from 21 oC to 17 oC.

Which type of reaction has occurred and why?

type of reaction reason

A exothermic heat is released

B exothermic heat is absorbed

C endothermic heat is released

D endothermic heat is absorbed








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EXAM QUESTIONS

QUESTION 3?The energy level diagram below shows the relative energies of the reactants and products in a reaction.

Which row correctly describes the type of reaction and corresponding energy change?

type of reaction energy change

A endothermic heat is released

B endothermic heat is absorbed

C exothermic heat is released

D exothermic heat is absorbed

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Exothermic & Endothermic Reactions